1. Field of the Invention
This invention relates to a laminate or multilayer polymeric film structure which is useful as a substantially liquid impermeable moisture vapor permeable membrane. In particular, the invention relates to a multilayer polymeric film structure comprising a substrate layer, a tie layer and a layer comprising a copolyetherester, wherein the structure has differential permeability.
2. Description of the Related Art
The outer walls and roof of a building usually include a layer of an insulation material. Further, wood is still commonly used in the construction industry particularly in the construction of buildings such as houses and the roofs of houses. The transmission of moisture vapor between the interior and exterior of buildings may result in the condensation of moisture vapor, and hence the build-up of deposits of moisture, in or on the insulation material and wood-containing elements of the building, which can cause considerable damage thereto. It is therefore of particular importance to prevent moisture build-up from coming into contact with the insulation materials and wood-containing elements in a building in order to keep them as dry as possible. In many countries, there exist building regulations which control this aspect of construction.
Polyolefin microporous membranes are of use in the construction industry, for example as roof or wall liners. Under all climatic conditions, it is desirable to control moisture vapor transfer across the walls or the roof of a building to prevent moisture build-up. Usually such membranes having a defined moisture vapor transmission rate (MVTR) are used to line the insulation material in buildings and are designed to control moisture vapor transfer in the building in winter, when the moisture vapor transfer is from the interior of the building to its exterior.
So-called breathable fabrics composed of a film of a polymeric material that is permeable to moisture vapor bonded to a textile material are also known. A notable and successful material that transmits moisture vapor therethrough is a film of microporous polytetrafluoroethylene that is adhered to a textile material. Although this product has been very successful, it is rather expensive and the pores tend to be blocked by dirt, body oils and detergents. It is known that other polymers can be formed into films that have a high moisture vapor transmission rate (MVTR) and, when covered with textile materials such as nylon or poly(ethylene terephthalate), can be used to make waterproof and water vapor permeable garments. U.S. Pat. No. 4,493,870 discloses waterproof garments made of a textile material covered with a single layered film of a copolyetherester made from a dicarboxylic acid, an aliphatic diol and a poly(alkylene oxide)glycol wherein at least 70% of the glycol has a carbon to oxygen ratio of 2.0-2.4. Such waterproof garments described therein have MVTR values that do not depend on which surface of the film faces the high humidity side. The values obtained are equal when either side is exposed to the same level of humidity.
EP-A-0611037 discloses a process for making a laminate usable in protective clothing, diapers, and roof underliners. In the process, a moisture vapor permeable, liquid impermeable, barrier layer with a thickness of 3 to 25 μm is coextruded with a 1 to 5 μm thick release layer on one side of the barrier layer and a 1 to 5 μm thick tie layer on the opposite side of the barrier layer. The tie layer is adhered to a porous substrate such as a woven or nonwoven fabric. The tie layer typically comprises a thermoplastic such as an ethylene copolymer or a polyurethane and serves to improve the adherence between the porous substrate and the breathable thermoplastic barrier layer.
U.S. Pat. No. 4,725,481 discloses a waterproof water vapor permeable film for use as surgical drape and in waterproof garments having rapid transmission of moisture vapor through the film toward the exterior or weather-side of the garment, while minimizing the transmission of water in the opposite direction, making the garment more comfortable to wear due to the increase in the MVTR away from the wearer while protecting the wearer from water, liquid and vapor from exterior sources. In particular, U.S. Pat. No. 4,725,481 discloses a bicomponent film of a hydrophobic layer and a hydrophilic layer of copolyetherester elastomers bonded together which permits differential transfer of moisture vapor to prevent buildup of moisture, the bicomponent film having a separation ratio for moisture vapor of at least 1.2 as determined by ASTM E96-66 (Procedure BW).
The separation ratio for moisture vapor refers to the MVTR measured with the hydrophilic layer of the bicomponent film next to the water surface divided by the MVTR of the bicomponent film with the hydrophobic layer next to the water surface, as described in ASTM E96-66 (Procedure BW), run at 22° C. The bicomponent film of U.S. Pat. No. 4,725,481 has a much higher MVTR, as measured by ASTM E96-66 (Procedure BW), when moisture vapor passes in the direction of the hydrophilic layer and then through the hydrophobic layer of the bicomponent structure, as contrasted to the passage of moisture vapor from the hydrophobic layer and then through the hydrophilic layer. These bicomponent films behave like a permeability valve. The permeability of the bicomponent film is not linear with vapor pressure (relative humidity). As the relative humidity is increased, the hydrophilic layer absorbs water in an amount determined by its composition which causes it to swell and become more permeable. The water swell capability of the copolyetherester increases with an increase in the weight percent of the long-chain ester units in the polymer. As a consequence, when the hydrophilic layer is next to the water source, the value of the MVTR is about two to three times higher than when the hydrophobic layer is next to the water source.
The use of waterproof moisture vapor permeable membranes in the construction industry is problematic since the materials suitable for such membranes are often incompatible with the base material or substrate, which is often made of a polyolefin. In other words, it is often not possible to provide adequate adhesion between these two layers such that the laminate product has a high resistance to delamination. This is especially the case when it is desired to produce a laminate having a thin water permeable membrane. In addition it is particularly difficult to maintain the integrity of the mechanical bond between the waterproof moisture vapor membrane and the substrate in a high moisture environment, since the waterproof moisture vapor permeable membrane can swell up to 40%.
Further, while waterproof moisture vapor permeable membranes may successfully control moisture vapor transfer in buildings in winter, those membranes do not work in the summer in regions where the climatic conditions reverse the vapor flow so the moisture vapor transfer is from the exterior of the building to its interior. Rather, under those conditions the membranes cause an undesirable moisture build-up in the roof or wall cavity of the building. Typically, such climatic conditions exist in semi-tropical regions, which have high temperature and humidity in the summer and low temperatures, typically well below 0° C., in the winter.
Laminated structures are almost exclusively manufactured by a process which involves the application of heat and/or pressure, such as a melt extrusion coating process or a conventional lamination process. It is considered that one reason for the poor adhesion of incompatible polymer resin and substrate combinations, especially when thin membranes are required, is that the molten polymer resin coating may cool too rapidly to allow for sufficient time for it to interact with the surface of the substrate and create strong adhesion. There must generally be sufficiently high penetration of the molten polymer resin coating into the interstices and porous structure of the substrate to ensure a good bond. In addition, rapid cooling of the polymer resin coating may cause the polymer coating to solidify before forming a layer of consistent thickness and this is, again, especially a problem when thin membranes are required. It is considered that, typically, the adhesion between an incompatible polymer resin coating and substrate consists predominantly of mechanical bonding, with little or no chemical bonding.
It is an object of this invention to provide a substantially liquid impermeable moisture vapor permeable membrane which has good adhesion between the substrate and polymer coating layer, particularly a thin polymer coating layer, and particularly a membrane having differential permeability.